The first purpose of the study is to look at the effect of giving 3 and 4 study drugs together on the amount of HIV virus in children's blood. The combination of 3 drugs is zidovudine (ZDV), lamivudine (3TC), and nevirapine (NVP). The combination of 4 drugs is zidovudine (ZDV), lamivudine (3TC), nevirapine (NVP), and 1592U89. The goal is to see if subjects will have an undetectable amount of HIV virus in the blood by week 12. A second purpose is to see how long after week 12 the amount of virus in the blood remains undetectable. A third purpose is to determine the safety of giving the 3- and 4- drug combinations. Zidovudine and lamivudine are approved by the FDA for use in children. Nevirapine has been studied inn about 475 children, but has not yet been approved by the FDA. 1592U89 has been studied in 21 children and it has not yet been approved. The 3-drug combination (zidovudine,lamivudine and nevirapine) has never been studied together in children. The 4-drug combination (zidovudine,lamivudine, nevirapine, and 1592U89) has never been studied together in children.